Throughout the world the multiple use of hypodermic syringe products which are intended for single-use only contributes to the transfer of contagious diseases. Intravenous drug users who routinely share and re-use syringes are a high-risk group with respect to the AIDS virus. Also, the effects of multiple use of a single syringe are a major concern in some countries where repeated use of syringe products during mass immunization programs may be responsible for the spread of many diseases.
Many attempts have been made to prevent reuse of syringes. Early efforts have utilized a specific act to destroy the syringe after use either by using a destructive device or by providing a syringe assembly with frangible zones so that the syringe could be rendered inoperable by an application of force. Other attempts have involved including structural elements which would allow the destruction or defeating of the syringe function by a conscious act by the syringe user. Although many of these devices work quite well, they do require the specific intent of the user followed by the actual act to destroy or render the syringe inoperable. These devices are not effective with a user having the specific intent to re-use the hypodermic syringe. Accordingly, there was a need for a single-use hypodermic syringe which, after use, will become inoperable or incapable of further use automatically without any additional act on the part of the user. The automatic function is much harder to provide because the means for rendering the syringe inoperable must not prevent its filling or use under normal conditions.
An example of a single-use syringe which automatically disables after injection is taught in U.S. Pat. No. 4,973,310 to Kosinski. This syringe contains a locking element positioned in the syringe barrel between the plunger rod and the inside surface of the barrel. In use, the syringe allows the user to draw a pre-selected amount of medication into the chamber of the barrel and deliver this medication, as through injection, into the patient. Any attempt to withdraw the plunger to use the syringe a second time will cause the locking element to embed itself into the inside surface of the syringe barrel to prevent proximal motion of the plunger rod.
Despite the availability of single use syringes with disabling mechanisms, there is still a need for improvements to a single-use syringe which will allow a pre-selected number of plunger rod strokes before the automatic disabling mechanism activates. For example, four strokes of the plunger may be required to complete the injection process, when the syringe assembly is used to draw a diluent into the syringe barrel, dispense the diluent into a vial containing the substance to be reconstituted, draw back the reconstituted medication into the syringe and then deliver the contents of the syringe into the patient. It would be desirable to provide such a syringe assembly that is simple and inexpensive to manufacture.